In terrestrial mobile radio systems of cellular, PCS or other type, geographical areas are subdivided into a number of cells. The communications traffic in each cell is supported by a base station and each base station has assigned to it a multiplicity of RF carriers. In such cellular mobile radio systems that operate at UHF and higher frequencies, the size of the cells is determined by terrain features (there can not be major obstructions between the mobile station and the base station), network capacity requirements (the number of users the system needs to support), and the base station receiver sensitivity (limited by losses and noise generated in the base station receiver front end). In such cellular mobile radio systems, cells are called capacity cells when their size is determined by traffic requirements, and cells are called coverage cells when their size is determined by the base station receiver sensitivity and the terrain. Furthermore, a distinction is made between the forward link, which is the radio signal transmitted from the base station to the mobile station, and the reverse link, which is the radio signal transmitted from the mobile station to the base station.
In the reverse link, the mobile station typically transmits 10 to 100 times less power than the base station transmits in the forward link. Therefore,the received signal strength at the base station is much lower than the received signal strength at the mobile station. In situations where the base station range is limited by the reverse link signal strength, the base station is identified as reverse link limited. Likewise, in forward link limited cells the range is limited by the strength of the signal received at the mobile station.
Mobile radio networks are designed for balanced forward and reverse links, i.e., equal base station range in both directions. This balance is based on the assumption that all cells are at full capacity. However, many operational networks are not at full capacity. Under these conditions, the base station transmitter can be driven harder to provide an increased range for the forward link. The cell is then reverse link limited.
Specialized Mobile Radio (SMR) base stations and rural cellular base stations are typically reverse link limited. In particular, many existing cellular base stations are reverse link limited because they were designed for car phones transmitting at about 8 Watts, while the majority of mobile stations today are battery operated hand-held phones, which transmit at much lower power levels (0.6 Watt in the US and 2 Watts in Europe).
Reverse link limitations in specific existing cells due to terrain can be overcome by increasing the antenna tower height at the base station. More general, construction of additional base stations or repeater sites is necessary. Both these approaches have major disadvantages: increasing the height of the receive antennas on the tower is typically not possible without replacing the entire tower and may violate zoning regulations. Building additional base stations or repeater sites is expensive and also requires a reassignment of the frequency reuse pattern of the network.
In capacity limited cellular networks, additional demand in the number of users can be met by adding new frequency channels to the existing cell sites if the additional channels are available. In networks where all channels are in use the only solution is splitting existing cells into smaller ones, and correspondingly, adding additional base stations and reassigning the frequency reuse pattern.